The present invention relates to electrostatic discharge protection in matrix addressable displays.
Flat panel displays are widely used in a variety of applications, including computer displays. One suitable flat panel display is a field emission display. Field emission displays typically include a generally planar emitter substrate covered by a display screen. A surface of the emitter substrate has formed thereon an array of surface discontinuities or xe2x80x9cemittersxe2x80x9d projecting toward the display screen. The emitters are conical projections which may be integral to the substrate. Typically, contiguous groups of emitters are grouped into emitter sets in which the emitters in each emitter set are commonly connected.
The emitter sets are typically arranged in an array of columns and rows, and a conductive extraction grid is positioned above the emitters. The extraction grid includes small openings into which the emitters project. All, or a portion, of the extraction grid is driven with a voltage of about 30-120 V. Each emitter set is then selectively activated by applying a voltage to the emitter set. The voltage differential between the extraction grid and the emitter sets produces an electric field extending from the extraction grid to the emitter set having a sufficient intensity to cause the emitters to emit electrons.
The display screen is mounted directly above the extraction grid. The display screen is formed from a glass panel coated with a transparent conductive material that forms an anode biased to about 1-2 kV. The anode attracts the emitted electrons, causing the electrons to pass through the extraction grid. A cathodoluminescent layer covers a surface of the anode facing the extraction grid so that the electrons strike the cathodoluminescent layer as they travel toward the 1-2 kV potential of the anode. The electrons striking the cathodoluminescent layer cause the cathodoluminescent layer to emit light at the impact site. Emitted light then passes through the anode and the glass panel where it is visible to a viewer. The light emitted from each of the areas thus becomes all or part of a picture element or xe2x80x9cpixel.xe2x80x9d
The brightness of the light produced in response to the emitted electrons depends, in part, upon the rate at which electrons strike the cathodoluminescent layer. The light intensity of each pixel can thus be controlled by controlling the current available to the corresponding emitter set. To allow individual control of each of the pixels, the electric potential between each emitter set and the extraction grid is selectively controlled by a column signal and a row signal through corresponding drive circuitry. To create an image, the drive circuitry separately establishes current to each of the emitter sets.
To produce the intense electric field that extracts electrons from the emitters, the openings into which the emitters project are very small. Consequently, the distances between the emitters and the grid sections are very short. If the voltage differential between the emitters and the grids is too high, electrons will be extracted from the emitters at a rate that is sufficient to damage the emitters. Such high differential voltages can occur during packaging and handling due to statically induced charge on either the emitters, the extraction grid or the anode.
A field emission display includes an electrostatic discharge (xe2x80x9cESDxe2x80x9d) circuit coupled to discharge statically induced charge, thereby reducing damage to the field emission display. In one embodiment of the invention, the field emission display includes an emitter substrate having a plurality of emitters formed thereon and an extraction grid formed from a plurality of grid sections adjacent to the emitter substrate. The ESD circuit is coupled between the grid sections and the emitter substrate to provide a current path to discharge statically induced charge when the voltage differential between the grid section and the emitter substrate exceeds a selected voltage. The ESD circuit preferably includes diodes having their anodes coupled to the emitter substrate and cathodes coupled to the grid sections.
In another embodiment of the invention, the ESD circuit includes a first portion coupled between the grid sections and a first reference potential and a second portion coupled between the emitter substrate and a second reference potential. The first portion is formed from a plurality of column protection diodes and the second portion is formed from a plurality of row protection diodes. In this embodiment, the first portion of the ESD circuit discharges statically induced charge when the voltage differential between the grid section and the first reference potential exceeds a selected first voltage. The second portion provides a current path to discharge statically induced charge from the emitter substrate when the voltage differential between the emitter substrate in the second potential exceeds a second selected voltage.
In one embodiment of the invention, the ESD circuit is formed from pn junctions integrated into the emitter substrate. In another embodiment of the invention, the ESD circuit is formed from pn junctions formed within an insulative layer carrying the grid sections.
In another embodiment of the invention, the field emission display also includes an ESD diode coupled between a transparent conductive anode on the display screen and a reference pad. The ESD diode has a breakdown voltage that exceeds the expected operating voltage of the transparent anode, so that the ESD diode only discharges the transparent anode when the voltage of the transparent anode is above its expected operating voltage.